A primary goal of the Durban Platform negotiations should be to develop an agreement that will maximize reductions in greenhouse gas emissions over time. Achieving this objective will be a function of not only the ambition of the 2015 agreement, but also the levels of participation and compliance by states. A higher level of ambition will not necessarily make the agreement more effective, if fewer states participate or comply.

In general, international agreements can serve a contractual, prescriptive, or facilitative function. In the climate change context, the contractual and prescriptive models are not politically realistic at this time, so the 2015 agreement should focus on the law's catalytic and facilitative roles.

Although the Durban Platform negotiations still have a long way to go, the decision adopted last year at COP-19 in Warsaw suggests that the 2015 agreement will have a hybrid quality, which seeks to balance national flexibility and international discipline. In many if not most countries, the climate change issue is driven more by national than by international politics, so the agreement needs to allow states to determine the content of their own commitments. This approach represents a concession to political and diplomatic realities, as well as to the limits of international agreements in influencing countries' behavior in an area so vital to their interests.

At the same time, the 2015 agreement needs to prod states to do as much as possible, through multilateral rules on transparency and accountability that help foster a virtuous cycle, in which states make progressively more ambitious contributions. Thus far, the top-down elements of the hybrid approach remain largely an abstraction. What remains to be seen is whether parties will be able to agree on rules that sufficiently discipline national flexibility and promote stronger ambition.